The present invention pertains generally to semiconductor lasers and more particularly to distributed feedback lasers.
The first semiconductor laser operated only at cyrogenic temperatures. Lasing action occurred in a planar p-n junction created in a gallium-arsenide crystal. Although the single p-n junction was not, strictly speaking, an optical waveguide, there was gain in the plane of the junction and high absorption losses outside of the junction so that the light beam emerged in the plane of the junction. Recently, room temperature semiconductor lasers have been fabricated using a double heterojunction structure with a higher index of refraction material, such as gallium-arsenide, surrounded by a lower index of refraction material, such as gallium-aluminum-arsenide. Optical gain occurs in the center of the gallium-arsenide layer, providing room temperature operation with good optical confinement.
A double heterojunction laser dedicated at room temperature operation must be grown by tedious deposition of several layers by liquid epitaxy. This method of fabrication, and the subsequent attachment of leads, causes strains and micro-cracks that form regions of high absorption and ultimately, limits the lifetime of the device to a few thousand hours. Expensive and delicate techniques are required to avoid creation of these flaws. The output power is limited since lasing occurs only in the small volume of the single p-n junction. The beam emitted is geometrically poor, being planar, fan shaped; and the beam divergence is poor due to the diffraction caused at the single, narrow emitting junction.
For the benefit of those not familiar with advances made in this art over the past decade, the ensuing brief bibliography is included.
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By the term "light," as used herein, is meant electromagnetic radiation not only in the visible spectrum but also in the infra-red and ultra-violet regions of the electromagnetic spectrum. The noun "semiconductor" is used to indicate an electronically conducting medium with a resistivity in the range between those of metals and insulators, and in which the conduction is by electrons, and holes.